Polar vortex expands southward
1. Almost three years ago, the term "polar vortex" went viral. Actually, some form of polar vortex is seen on weather maps throughout the year. In Northern Hemisphere's summer, it is often a small feature having zero impact on U.S weather. However, it became a major issue during a protracted period of cold weather in the Midwest and Northeast in the winter of 2013-2014. Temperatures ran 7.5 degrees below average for the three months from December through February. Despite the persistence of the cold, only one new record low temperature was reached: -16 on Jan. 6. In fact, so far this century, that has been the only January record low so far this century. In the same span of years, five new record highs have been established in that month. The following season included an especially cold February. In 2015, February was 13.1 degrees below average in Chicago, and two new record lows were set... the first ones of this century. This past February was much milder, however, with temperatures for the month 15.6 degrees higher than the year before.
2. Why the cold talk? What about the polar vortex? This week will turn colder, with temperatures likely staying in the single digits in Chicago on Thursday and below freezing in New York City from Wednesday night until Saturday afternoon. These three maps show the forecast upper air patterns for today, Thursday and Saturday. You can see how the flow changes to become very cold, then changes back. The Thursday map has the vortex extending southward.
On this map for today, A is the upper air trough supporting the storm now leaving the East Coast. B is supporting the Arctic front in west-central Canada.
Snow flurries and squalls will be common at this time downwind from the Great Lakes and into the mountains.
The southwesterly flow show here could support a rapidly moving low pressure area that would more likely bring rain and than to the major Northeast coastal cities.
3. Weak disturbances embedded in the flow could produce a round of mainly light snow from Illinois to Pennsylvania late tonight (west) and later tomorrow (east). Snow showers will accompany and follow the Arctic cold front 24-36 hours later. Extensive lake effect will break out once again, and with the extreme cold and blowing snow, it could be a dangerous situation in lake-effect country. Fortunately, many people in those areas routinely pack a winter survival kit in their vehicles. Google "winter survival kit for car list" to see what I mean.
4. During the days before and right after Christmas, it appears that a fast changing flow pattern can dominate the Central and Eastern states, bringing alternating cold shots and warmups. The exact positioning of weather systems will determine how the weather is for you Christmas Eve and Christmas Day.
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